1. Field of the Invention -
This invention relates to the formation of resinous coatings on metallic surfaces. More specifically, this invention relates to the deposition of improved resinous coatings without need for reaction rinses on metallic surfaces by contacting the metallic surfaces with an acidic aqueous coating solution containing dispersed solid resin particles.
Autodeposition involves the use of an aqueous resinous coating composition of low solids concentration (usually less than about 10%) to form a coating of high solids concentration (usually greater than about 10%) on a metallic surface immersed therein, with the coating increasing in thickness or weight the longer the time the metallic surface is immersed in the composition. Autodeposition is similar to electrodeposition, but does not require the aid of external electrical current to cause the resin particles to deposit on the metal surface. In general, autodepositing compositions are aqueous acid solutions having solid resin particles dispersed therein.
2. Statement of the Related Art
Acidic aqueous coating solutions having dispersed therein solid resin particles and having the capability of forming on metallic surfaces immersed therein resinous coatings which grow with time are disclosed in various patents.
Autodeposited coatings are disclosed, for example, in U.S. Pat. Nos. 3,585,084; 3,592,699; and 4,373,050, each to Hall and Steinbrecher. The autodepositing compositions disclosed in these patents are aqueous solutions of acid and oxidizing agent, with solid resin particles dispersed therein, particularly a latex combined with hydrofluoric acid and hydrogen peroxide. U.S. Pat. No. 3,709,743 discloses a similar composition comprising an acidic aqueous solution of dispersed resin solids in which the acid component is nitric acid. U.S. Pat. Nos. 4,347,172 and 4,411,937 disclose an improved autodepositing composition comprising hydrofluoric acid, ferric iron, for example, ferric fluoride, and dispersed resin solids. In this process, an oxidizing agent such as peroxide is disclosed as an optional ingredient.
In accordance with the disclosures of the aforementioned patents, autodeposited coatings of the type therein are treated with a solution of chromium compounds prior to curing in order to impart to the coatings corrosion resistant properties which are of an acceptable nature. Such chrome treatments are disclosed in U.S. Pat. Nos. 3,647,567; 3,795,546; and 4,030,945.
Autodeposited coatings with better corrosion resistant properties (up to about 1000 hours of salt spray resistance) which are achieved with a chrome after treatment are disclosed in U.S. Pat. No. 4,313,861. This patent discloses that such improved autodeposited coatings are based on the use in autodepositing compositions of particular acrylic copolymers as the resin component. The acrylic copolymers have a Tg of 2.degree. to 50.degree. C. and are prepared from either methacrylic acid or acrylic acid and at least one other polymerizable ethylenically monounsaturated monomer, and optionally, hydroxyl-containing monomer and/or a vinylenically polyunsaturated monomer.
It has also been recognized that the effectiveness of autodepositing compositions is influenced by surfactant and gegenion concentrations and that the corrosion resistance of autodeposited coatings can be improved by the use of particular resin dispersions. For example, U.S. Pat. No. 4,191,676 discloses an autodepositing composition containing a dispersion of polymer particles in which the surfactant concentration in the aqueous phase is below the critical micelle concentration. A particular class of resins for use in such compositions is prepared by polymerizing: (i) 25 to 70 wt. % of a conjugated diene, for example, butadiene; (ii) 5 to 70 wt. % of a CH.sub.2 =CHR compound in which R is an aryl or a cyano group, for example, styrene and acrylonitrile; (iii) 1 to 50 wt. % of a vinylidene halide, for example, vinylidene chloride; and (iv) a monoethylenically unsaturated monomer having a functional group, for example, acrylic acid and methacrylic acid. Autodeposited coatings formed from compositions within the scope of the disclosure of this patent and treated with a chromium-containing solution exhibit good corrosion resistance after 240 hours of exposure in standard salt spray tests. U.S. Pat. No. 4,180,603 discloses a coating composition containing epoxy resin solids and a cross-linking resin which when used without a chrome after-treatment produces coating capable of withstanding up to 336 hours of exposure in standard salt spray tests.
Additional, U.S. Pat. Nos. which relate to autodeposition coating include: 3,063,877; 3,776,848; 3,791,431; 3,936,546; 4,108,817; 4,177,180; 4,186,219; and 4,318,944. The foregoing additional patents disclose the use of various polymer latices in autodeposition baths, including acrylates and styrene copolymers.
British patents 1,538,911 and 1,559,118 disclose the use of latices containing up to 55% by weight of vinylidene chloride monomer.
European published patent application 71,355 also discloses autodeposition bath latices containing 70-95% vinylidene chloride and at least one other monomer. The other monomers include: lower alkyl acrylates and methacrylates; their derivatives; alkyl nitrile; vinyl chloride; acrylamide; acrylamide derivatives; vinylsulfonic acid, its salts, and esters; acrylic acid; methacrylic acid; and itaconic acid. However, this publication contains no information regarding the quantity of emulsifier to be used, and there is therefore no teaching of the internal or external stabilization of the resulting vinylidene chloride copolymer. The quantities of emulsifier normally employed in the art are described in the Encyclopedia of Polymer Science and Technology, J. Wiley and Sons, Inc. (pub.), New York (1971) at 14:551,553. 1.01% emulsifier is suggested for a latex with 35% solids, and 1.48% emulsifier is suggested for a latex with over 50% solids. H. Barth in Methoden der Organischen Chemie, Houben-Weyl, Thieme Verlag (pub.), Stuttgart, Germany (1961) at 14/1:900 specifies an emulsifier quantity of 0.74%. In none of the above instances, is an internally or externally stabilized vinylidene chloride copolymer produced.